TLP Pontoon

ABSTRACT

A TLP design with improved motion characteristics and that is drawn to a means of reducing the required tendon stiffness and thereby reducing the overall cost of deepwater TLPs. The invention reduces the hydrodynamic added mass of the TLP hull. The horizontal pontoons that connect the vertical columns of the TLP are shaped to reduce the hydrodynamic added mass of the structure in the vertical direction.

FIELD AND BACKGROUND OF INVENTION

The invention is generally related to offshore floating structures and,more particularly, to a TLP (tension leg platform).

TLPs are floating structures permanently moored to the seafloor byvertical mooring members, called tendons (FIG. 1). Tendons restrain theplatform in such a way that heave, pitch, and roll motions are small.Small vertical motions allow the platform to support vertically arrangedtop-tension risers (TTRs). For application in deepwater the length ofthe tendons have to increase, which adversely affects the dynamicbehavior of a TLP and also increases the costs. For these reasons, TLPsbecome less attractive with increasing water depth.

A TLP moored by its vertical tendons represents the dynamic systemdepicted in FIG. 2, which is a mass-spring representation of a TLP andits tendons. It has an effective mass M_(eff), and an effective elasticvertical stiffness C_(eff). The majority of the vertical stiffness isprovided by its tendons. Only a small stiffness contribution comes fromthe hydrostatic stiffness due to the hull's water plane area. Theeffective mass of a TLP is composed of the total body mass of hull andtopside, the hydrodynamic added mass of the surrounding water, and aportion of the tendon mass.

The system in FIG. 2 will oscillate following an excitation by animpulsive load. The cycle period of the ensuing oscillation is calledthe natural period. The natural period of the system in FIG. 2, T_(n),can be calculated by equation 1 below.

$\begin{matrix}{T_{N} = {2{\pi \cdot \sqrt{\frac{M_{eff}}{C_{eff}}}}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

The natural period of a TLP is an important property since it influencesthe TLP's dynamic response to ocean waves. In the TLP's nominal positionthe buoyancy of the hull keeps the tendon under constant tension. Whenexposed to ocean waves, a TLP undergoes dynamic motion response whichgives rise to fluctuating tendon tensions. If the tendon tensionfluctuations become too large, the tendons may fail. A primary objectivein TLP design is therefore to keep the dynamic tendon loads withinacceptable limits.

The magnitude of a TLP's dynamic response to waves is determined by themagnitude of the exciting load and by the ratio between the excitationperiod to the natural period of the TLP. The response is largest whenthe period of the wave excitation is equal to the natural period of theTLP. The dynamic response becomes smaller when the natural period iswell separated from the period of excitation. A fundamental designprinciple for TLP design is therefore to keep the vessel's naturalperiods well outside from the wave energy range.

Ocean waves are typically composed of a series of waves wherebysignificant energy is contained in waves with periods between about 5and 25 seconds. TLPs are therefore designed to have their naturalperiods outside the wave energy range, i.e. below about 5 seconds andabove 25 seconds, as indicated in FIG. 3.

Keeping a TLP's natural periods for heave, pitch, and roll below thewave energy range becomes increasingly difficult when the water depthincreases. The challenge stems from the fact that a tendon's axialstiffness decreases when it gets longer. As seen from equation 1 above,decreasing tendon stiffness causes the natural periods of the TLP toincrease and thereby to encroach on the wave energy range.

The axial stiffness of a single tendon is determined by equation 2 belowwhere C_(Tendon) is the axial stiffness of the tendon, E is the elasticmodulus of the tendon material, A_(eff) is the effective cross sectionalarea of the tendon, and L is the length of the tendon.

C _(Tendon) =E·A _(eff) /L  Equation (2)

It can be seen from equation 2, as the length L of a tendon increases,its axial stiffness decreases.

In order to counter the effect of reduced tendon stiffness in deeperwater, either the size or the number of the tendons has to be increased.The additional tendon weight then also requires a larger hull. As aresult, the overall cost of TLPs increases significantly with waterdepth.

SUMMARY OF INVENTION

The present invention mitigates the adverse effects referenced above andis drawn to a means of reducing the required tendon stiffness andthereby reducing the overall cost of deepwater TLPs. The inventionreduces the hydrodynamic added mass of the TLP hull. The horizontalpontoons that connect the vertical columns of the TLP are shaped toreduce the hydrodynamic added mass of the structure in the verticaldirection.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming partof this disclosure. For a better understanding of the present invention,and the operating advantages attained by its use, reference is made tothe accompanying drawings and descriptive matter, forming a part of thisdisclosure, in which a preferred embodiment of the invention isillustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, forming a part of this specification, andin which reference numerals shown in the drawings designate like orcorresponding parts throughout the same:

FIG. 1 is a schematic illustration of a TLP.

FIG. 2 is a Mass-Spring representation of a TLP and its tendons.

FIG. 3 is a graph of a typical wave energy spectrum.

FIG. 4 is an illustration of a TLP with four columns and four pontoonswith extensions.

FIG. 5 provides examples of pontoon cross sections.

FIG. 6 depicts pontoon added mass vs. cross section height-to-widthratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a typical TLP 10 which includes columns 12, pontoons14, deck 16, and tendons 22. The TLP hull is the combination of thecolumns and pontoons and, if present, pontoon extensions. As seen, thecolumns 12 support the deck 16 above the water and the pontoons 14 arerigidly attached to the columns 12 to hold them in their spaced apartrelationship and may provide buoyancy to the columns 12 and deck 16. Theupper ends of the tendons 22 are attached to the pontoons 14 or columns12 and the lower ends of the tendons 22 are anchored to the sea floor tohold the TLP in the desired position for drilling and/or productionoperations.

From equation 1 above it can be seen that the natural period is not onlydetermined by the effective stiffness but also by the effective mass,M_(eff). If the mass is reduced by the same rate as the stiffness isreduced, the natural period remains unchanged. Light weight design istherefore of increasing importance for deepwater TLPs.

Another way to reduce the effective mass in equation 1 is to reduce thehydrodynamic added mass of the hull. As stated above, a portion of thetotal effective mass is contributed by the hydrodynamic added mass dueto the water surrounding the hull.

The hydrodynamic added mass of a TLP is typically in the same order ofmagnitude as the vessel's displacement. It varies for different hullshapes and is expressed by an added mass coefficient C_(a). An addedmass coefficient of 0.8 indicates that the added mass of a hull is 80%of its displaced water mass.

The present invention is directed to a particular shape of the TLP hull,more specifically the pontoons, to reduce the hydrodynamic added mass.

FIG. 4 illustrates a TLP 10 with four columns 12 and pontoons 14connecting the columns 12 together. The pontoons 14 span the lower endof the columns 12 and are rigidly attached to the columns 12. A deck 16is attached at the upper end of the columns 12 and is above the waterline during normal operations offshore. The deck 16 normally includesliving quarters as well as production and/or drilling equipment notshown. The TLP hull may also have pontoon extensions 20 extendingoutwardly from the columns 12, providing additional buoyancy andstability. It should be understood that the TLP drawing is only oneexample of a TLP configuration and that more or fewer columns may beused.

FIGS. 5 A-D illustrate examples of different cross sections of pontoons14. The hydrodynamic added mass coefficient of a pontoon is dependent onthe shape of the pontoon cross section. FIG. 6 depicts the hydrodynamicadded mass coefficient in the vertical direction for a rectangularpontoon cross section (i.e., FIG. 5 b). As seen in FIG. 6, a largerheight-to-width ratio of the pontoon cross section leads to a reductionof the hydrodynamic added coefficient. Selecting pontoons with largeheight-to-width ratios are therefore beneficial to keep the heave,pitch, and roll natural periods of a TLP separated from the wave energyrange.

Thus, FIG. 5 D illustrates the generally preferred type of pontoon crosssection for the use of TLPs in deeper water, as opposed to FIGS. 5 A andC where the height-to-width ratio is essentially one or FIG. 5 B wherethe height-to-width ratio is less than one. It may also be preferablethat the pontoon cross section have a semi-circular rounded top andbottom as seen in FIG. 5 D with a height-to-width ratio of at least 1.2.The semi-circular rounded top and bottom contribute to a reduction ofthe vertical hydrodynamic added mass.

While specific embodiments and/or details of the invention have beenshown and described above to illustrate the application of theprinciples of the invention, it is understood that this invention may beembodied as more fully described in the claims, or as otherwise known bythose skilled in the art (including any and all equivalents), withoutdeparting from such principles.

What is claimed as invention is:
 1. A floating tension leg platform foroffshore production and drilling, comprising: a. a deck; b. a pluralityof columns attached to and extending downwardly from the deck; and c.pontoons spanning the lower ends of the columns and rigidly attached tothe columns, with the pontoons having a height-to-width ratio of atleast 1.2.
 2. The TLP of claim 1, wherein each pontoon has asemi-circular rounded top.
 3. The TLP of claim 1, wherein each pontoonhas a semi-circular rounded bottom.
 4. A floating tension leg platformfor offshore production and drilling, comprising: a. a deck; b. aplurality of columns attached to and extending downwardly from the deck;and c. pontoons spanning the lower ends of the columns and rigidlyattached to the columns, with each pontoon having a height-to-widthratio of at least 1.2 and having a semi-circular rounded top and bottom.